Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, laptop computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. In this aspect, electrochemical devices have attracted the most attention, and among them, the development of rechargeable secondary batteries has been the focus of particular interest. In recent years, extensive research and development for new electrode and battery design is being conducted to improve the capacity density and specific energy of the batteries.
Among currently available secondary batteries, lithium secondary batteries developed in the early 1990's have received a great deal of attention due to their advantages of higher operating voltages and much higher energy densities than traditional batteries using aqueous electrolyte solutions, such as Ni-MH batteries, Ni—Cd batteries and H2SO4—Pb batteries. However, such lithium ion batteries have disadvantages of safety-related problems caused by the use of organic electrolyte solutions, for example, ignition and explosion, and complex manufacturing. Lithium ion polymer secondary batteries designed to overcome the weak points of lithium ion batteries are stated to be one of the next-generation batteries, but their capacity is still lower than that of lithium ion batteries and a discharge capacity, particularly, at low temperature, is insufficient, and accordingly, there is an urgent demand for improvement.
Such electrochemical devices are produced by many companies, but their safety characteristics show different aspects from each other. Assessing and ensuring the safety of electrochemical devices is very important. One of the most important considerations is that electrochemical devices should not cause damage to users in the event of malfunction, and for this purpose, Safety Standards impose strict regulations on ignition and explosion in electrochemical devices. In the safety characteristics of electrochemical devices, electrochemical devices have a high risk of explosion in the event of overheat or thermal runaway of an electrochemical device or penetration of a separator. Particularly, a polyolefin-based porous polymer substrate commonly used as a separator of an electrochemical device shows serious thermal contraction behaviors at the temperature higher than or equal to 100° C. due to material characteristics and procedural characteristics including stretching, causing a short circuit between a positive electrode (or cathode) and a negative electrode (or anode).
To solve the safety problem of electrochemical devices, a separator with a porous organic-inorganic coating layer formed by coating a mixture of excess inorganic particles and a binder polymer on at least one surface of a porous polymer substrate having plural pores was proposed. The inorganic particles included in the porous organic-inorganic coating layer have good heat resistance, thereby preventing a short circuit between a positive electrode (or cathode) and a negative electrode (or anode) when an electrochemical device is overheated.
Generally, a separator with a porous organic-inorganic coating layer is manufactured through a process which forms an organic-inorganic coating layer on a porous polymer substrate by dip coating. However, due to the use of an organic solvent-based slurry, this manufacturing method has safety hazards in the manufacture of an electrochemical device, and is less environmentally friendly and economically efficient.
As opposed to an organic solvent-based slurry, an aqueous slurry is safe, eco-friendly, and economically efficient, but its high surface tension causes a low wettability problem on a polyolefin-based substrate, limiting the use for separator coating.